This invention relates generally to the manufacture of fiber reinforced thermoplastic pipe lengths and more particularly to a novel continuous processing means for the manufacture of these articles.
The fiber reinforcement of pipe members formed with both thermoset and thermoplastic organic polymers has already gained wide commercial acceptance attributable to affording high strength and stiffness per unit weight when compared to pipe fabrication with the conventional materials previously used for transporting various fluids such as concrete, steel and the like. A variety of fabrication procedures are also well known to produce the composite pipe member with continuous fiber filaments which are commonly applied to the outer pipe surface. In one example, a tension winding process is commercially employed whereby a thermoplastic pipe or coupler member is rotated on a mandrel while the reinforcing fiber is being applied under heat and tension. The fiber tension during winding or wrapping around the underlying thermoplastic member provides a compaction force therebetween to secure thermal bonding of the fibers after the heating has melted or softened the outer surface of the thermoplastic material. This process has been found limited to relatively high fiber angles with respect to the longitudinal axis of the pipe or coupler member, typically greater than 15 degrees because the radial component of the fiber tension provides this compaction force. While tension winding can be augmented with a use of compaction roller means to increase the radial compaction force, the resulting low fiber angles produce undesirable fiber build-up at the mandrel ends since the continuous fibers being applied cannot be cut for a restart of the filament winding process. A further need to apply relatively high pressure as well as provide mandrel rotation during said tension winding process requires robust and expensive mandrels together with significant mandrel handling labor. In a different manufacturing process for the production of reinforced plastic pipe members, continuous glass fibers in a thermoset epoxy matrix are employed. Still other manufacturers produce a reinforced thermoplastic pipe construction having the thermoplastic pipe member as an innerlayer which is surrounded by various outerlayers of the reinforcement material. For example, one such manufacturer surrounds the thermoplastic liner with a fiberglass layer in an epoxy resin matrix and provides an exterior protective layer thereover of a still different fiber material which is again contained in an organic polymer matrix. Using thermoset polymers in the reinforcement of a thermoplastic pipe member frequently creates additional manufacturing problems. The curing required of these polymers occasions contamination as well as time delay and these materials are not recyclable.
Another complex reinforced thermoplastic pipe construction is disclosed in U.S. Pat. No. 4,850,395. As therein described, a core member of thermoplastic material is wound with an inner aramid fiber layer while being covered with still additional tape and metal outer layers. Such end product is understandably found to be both cumbersome and expensive to manufacture. In U.S. Pat. No. 4,469,138 there is disclosed polypropylene pipe lengths reinforced by simply mixing discrete carbon fibers in the starting polymer composition. The resulting product lacks the mechanical strength afforded with continuous fiber reinforcement as well as lacks the ability to orient continuous fibers in a predetermined spatial direction for maximum effectiveness in withstanding applied stress when the pipe member is in service. In the latter regard, such controlled directional orientation of the continuous fiber component in the reinforced thermoplastic pipe member enables the fiber placement to be fixed for such maximum effectiveness since the fiber materials being employed are generally stronger than the thermoplastic polymers forming the pipe member. The fiber reinforced end product is thereby only as strong as the spatial direction of the applied fibers with respect to the direction of the external stress when applied to said reinforced pipe member. Thus, when the fiber reinforced pipe member is stressed by internal fluid pressures in the direction of the fiber placement, the applied load is withstood primarily by the included fibers and the strength of said pipe member is at maximum value. Conversely, when the composite member is stressed in a perpendicular direction to the fiber direction, the applied force must necessarily be resisted primarily by the polymer pipe member so that pipe strength is at a minimum. From such consideration and a further analysis of the expected stress during pipe service employing recognized shell theory calculations, it has been determined that certain installations of the present fiber reinforced thermoplastic pipe members dictate a fiber orientation in the hoop direction whereas dissimilar pipe installations require the fiber direction to be oriented at lesser angles with respect to the longitudinal axis of the reinforced pipe member.
A still more arduous method for reinforcement of a thermoplastic pipe member is disclosed in U.S. Pat. No. 4,347,090 which employs a fabric sleeve applied about the pipe member for this purpose. The method requires an inner liner to be filled with liquid which is then heated causing said liner to expand as well as become partially molten for thermal bonding to an overlying glass fabric layer. An outermost layer comprising a thermoset resin impregnated glass cloth completes the reinforcement. Such final product and its method of fabrication are understandably both complex and expensive.
In U.S. Pat. No. 4,770,442 there is also disclosed a rather complex electrofusion type coupler being employed to join synthetic resin pipe lengths together. Said coupler member employs a cylindrical thermoplastic sleeve which includes a metal heating wire being disposed adjacent the inside surface while being reinforced on the outside surface with a winding or covering formed with a material composition exhibiting a lower thermal expansion than that of the thermoplastic sleeve material. Such reinforcement is said to limit any outward radial expansion of the composite sleeve during subsequent thermal bonding of said member to the pipe lengths being joined together by this means.
It is an important object of the present invention, therefore, to provide a more effective means for the fiber reinforcement of a thermoplastic pipe member in a continuous manner.
It is another important object of the present invention to provide a novel fiber reinforced thermoplastic pipe member having the fiber placement physically incorporated therein so as to better resist the applied stress encountered during use in a significantly improved manner.
Still another important object of the present invention is to provide a novel method for continuous fabrication of a fiber reinforced thermoplastic pipe member.
A still further important object of the present invention is to provide novel apparatus means for the continuous fabrication of a fiber reinforced thermoplastic pipe member.
These and still further objects of the present invention will become more apparent upon considering the following more detailed description of the present invention.
It has been discovered, surprisingly, that fiber reinforcement of a thermoplastic pipe length can be carried out more effectively in a continuous manner by reversing the customary relative rotation between the fiber when applied and the pipe member. More particularly, the processing procedure of the present invention continuously moves the pipe length in a linear direction without rotation while wrapping a plurality of continuous juxtapositioned reinforcement fibers formed with a material composition selected from the group consisting of ceramics, metals, carbon and organic polymers in an unbonded condition about the outer surface of said moving pipe member in a predetermined spatial direction and thereafter heating the fiber wrapped pipe length sufficiently to cause thermal bonding between the reinforcement fibers and the pipe outer surface while said fiber wrapped pipe length continues to move in the same linear direction. To impart increased mechanical strength for said fiber reinforcement requires that the fiber placement be carried out with the fibers being oriented in a particular spatial direction depending upon end use of the wrapped pipe length as previously pointed out herein. In accordance therewith, the fibers can be wrapped in a hoop direction about the pipe length as well as wrapped at lesser or interim angles with respect to the longitudinal axis of said pipe length. Additionally, multiple wraps of the reinforcement fibers can be applied continuously in accordance with the present processing procedure to include one or more overwraps being applied to serve as a protective covering from exposure of the final product to environmental or mechanical damage. Likewise, having the individual fiber wraps applied continuously in different spatial directions is contemplated in accordance with the present processing procedure and with the individual wraps all being bonded to the underlying thermoplastic pipe member after placement with a single heating step. The present processing procedure can similarly be carried out with multiple pipe lengths that have been joined together at the pipe ends in an in-line configuration before continuous fiber placement by employing conventional means such as butt-welding, adhesive bonding and the like as well as possibly being joined together with only a physical abutment existing therebetween. Suitable thermoplastic organic polymer materials forming the pipe member in the present composition construction include but are not limited to polyethylene such as high density polyethylene and medium density polyethylene, polypropylene, polyphenylene sulfide, polyetherketoneketone, polyamide, polyamideimide, and polvinylidene difloride. Similarly, a wide variety of materials are found suitable as the fiber reinforcement in the present processing procedure to again include but not be limited to ceramics, metals, carbon, aramid and other polymer fibers having softening temperatures above that of the pipe service temperature in use and glass compositions such as E type and S type glass.
Basically, the present thermal bonding of continuous fibers after having been wrapped in an unbonded condition about the outer periphery of the thermoplastic pipe length or lengths involves an operative cooperation between the applied fibers and the outer surface of said pipe member. A softening or melting action takes place during the thermal bonding step between the outer surface of the thermoplastic pipe member and any thermoplastic polymer materials serving as the matrix composition in selected preformed tape embodiments having the continuous reinforcement fibers bonded therein. In this manner, the applied fibers become physically joined to the pipe member with softened or melted thermoplastic polymer when the fiber wrapped pipe member is heated in the present method such as occurs when melting said outer pipe surface. The present heating step also produces a significant radial expansion of the pipe member upon heating which further promotes the physical adherence of the applied fibers to the outer pipe surface. In so doing, a radial compaction or compressive force is generated in the joined components with the maximum benefit being imparted by having the coefficient of thermal expansion for the selected fiber material being lower than that for the thermoplastic pipe polymer. The herein defined fiber reinforcement method understandably enables a wide variety of fiber materials to be selected as previously pointed out. Thus, a reinforcement fiber material can be selected from the aforementioned class of suitable materials so long as it is mechanically stiffer than the selected thermoplastic pipe polymer and has a glass transition or melting temperature higher than the service temperature of the thermoplastic pipe member during use. Selected polymer fibers can understandably include continuous bare filaments and comingled continuous fibers which can be wetted by polymer melt flow in the above mentioned compaction zone during heating. For selection of a suitable preformed continuous fiber material or prepreg tape having a matrix formed with a thermoplastic polymer, said matrix polymer is desirably chosen to have a softening or melt temperature equal to or lower than the softening temperature of the selected pipe polymer.
Any suitable heating source can be used in the present method to continuously bond the applied fiber reinforcement to the outer thermoplastic pipe surface. Contemplated heat sources include but are not limited to inert gases, oxidizing gases and reducing gases, including mixtures thereof, infrared heating sources, such as infrared panels and focused infrared means, conduction heating sources such as heated rollers, belts and shoe devices, electrical resistance heating sources, laser heating sources, microwave heating sources, RF heating sources, plasma heating sources and ultrasonic heating sources. An external flame heat source provides economical heating with high energy densities and with the gas burner or burners being suitably designed so as to surround the outer circumference of the fiber wrapped thermoplastic pipe member. Understandably, the employment of a continuous heating step in the present method can further advise auxiliary equipment means to be operatively associated therewith in the event of process interruptions such as fast responsive heating devices or cooling means averting meltdown of the materials being processed. It is also contemplated that the present method of thermally bonding the applied reinforcement fibers continuously to the outer surface of said thermoplastic pipe member can be modified in a still further manner. Accordingly, roller members rotatably mounted so as to press against the already heated fiber wrapped thermoplastic pipe member can be employed to help generate the aforementioned compaction force found beneficial in carrying out the present method. Such compaction rollers can be cooled, heated or remain at ambient temperatures in the present method depending upon the process requirements being carried out as well as the physical characteristics desired in the final product.
As above indicated, the present continuous method necessarily further includes provision being made for both start-up as well as termination and possible process interruption. Satisfactory start-up of the present processing procedure can be conducted in various ways to include starting with a single thermoplastic pipe length or multiple pipe lengths and feeding these members with continuous linear motion at a relative constant velocity to suitable fiber winding means. A continuous thermoplastic pipe length can be suitably provided employing conventional extruder means while the feeding of discrete pipe lengths to the operatively associated fiber winding means can preliminarily involve having the discrete pipe lengths simply butted together physically in an in-line configuration as well as having the respective pipe ends fused or welded to each other. Conventional means can be employed for continuous transport of the thermoplastic pipe member during processing in said manner including known moving belt drive mechanisms and motor driven rollers. Suitable start-up would also optionally include having the initial continuous fiber being physically secured to the outer surface of the first moving thermoplastic pipe member being processed with conventional clamping or adhesive bonding means. Following a continuous thermal bonding of the applied reinforcement fibers in the manner previously described, the present method can be terminated with conventional cut-off means being employed to suspend any further fiber placement. Satisfactory cut-off means can utilize a moving cutter device traveling in the same linear direction as the moving pipe member or members and to include a cutter severing the pipe along with the applied fiber reinforcement.
Novel apparatus means are employed to provide continuous fabrication of the present fiber reinforced thermoplastic pipe member. Basically, the present apparatus employs pipe feeding means which continuously transports the pipe length in a linear travel direction for operative association with rotating fiber supply means, fiber supply means which rotate about the circumference of said moving pipe length to continuously apply a plurality of juxtapositioned reinforced fiber wraps in a predetermined spatial direction on the outer surface of said moving pipe length, and heating means which causes thermal bonding to be continuously formed between the applied reinforcement fiber wraps and the outer surface of the moving pipe length. Said apparatus can further include a plurality of individual fiber supply means to serve various purposes such as applying successive fiber wraps at different fiber placement angles, reversing the application direction of successive fiber wraps, and applying overwraps of continuous fiber or tape material to serve as a protective covering against environmental or mechanical damage to the final product. A satisfactory embodiment for said individual fiber supply means can be a conventional cylindrical winder mechanism operatively associated with a rotary fiber or tape spool. The pipe feeding means in the present apparatus can also be of conventional construction, as previously pointed out, to include known moving belt drive mechanisms as well as motor driven rollers and the like. While a further previously mentioned wide variety of heating sources can be employed in the present apparatus for continuous thermal bonding of the applied fiber or tape wraps, it remains advisable for the selected heat source to uniformly provide heat about the entire circumference of the fiber wrapped pipe length in doing so. In the apparatus embodiments to be more fully described hereinafter, the particular heating means being employed consists of a stationary cylindrically shaped heater that surrounds the entire circumference of the fiber wrapped pipe member and which is equipped with appropriately disposed internal gas burners. Moving cutter means are also employed in the illustrated apparatus embodiments for the purpose of severing the moving fiber wrapped pipe member into one or more suitable lengths upon completion of the herein described thermal bonding procedure. The entire apparatus in said illustrated embodiments is further operated automatically with known robotic control means employing a conventional velocity servomechanism system.